The restoration of grossly broken-down crowns of teeth presents a significant clinical problem. Such teeth are restored by building new cusps from either amalgam or resin composites. These restorations may stand as either the final restoration in the case of amalgam or as a crown form onto which a gold casting may be seated. Since little natural dentin may be available for gaining mechanical retention, metal pins have traditionally been placed with remaining dentin to be used to regain the restoration. This research was designed to study a new method of retaining such restorations by using slots cut in the remaining natural dentin. The aims for this period were: 1) to complete gathering in vitro data on comparative efficacy of the slot and pin retained restoration, and 2) to design and start the clinical phase of the study. RESULTS: The specimens of composite retained by pins were significantly different from all other groups. At the 30 microns failure criteria the composite pin failed at a mean of 240 cycles under a normalized 70 Nt load, whereas the composite slots failed at 540,000 cycles. The composite slot data are comparable to the amalgam slot data. Twenty-two pairs of cusp-replacement amalgams were placed in twenty patients. One of each pair of amalgams had pin retention, the other had slot retention. Restorations varied from one to four-cusp replacement amalgams. Baseline evaluations were made by three dentists previously calibrated in clinical sessions. At baseline, all restorations were rated intact and acceptable in all evaluation categories. One year evaluations were scheduled.